Counting instances of individual derived classes

Question

I\'d like to be able to count instances of classes that belong in the same class hierarchy.

For example, let\'s say I have this:

class A;
class B: pu         


        

Answer 1

The current solutions all seem to count in constructors, and therefore also count in constructors of base type. (except Mykola, but that solution has an implicit conversion from bool - bad) This double-counting wasn't desired. The tricky part, though, is that during the construction of a derived type, the object temporary has intermediate base types. And with virtual base classes, this situation is even worse. In that case, the intermediate type isn't even well-defined.

Ignoring such transients, there are a few solutions to the double-counting:

• Use a flag to disable counting
• Use a protected base class constructor which doesn't count
• Decrease the instance count of your base classes in derived constructors '*(this->base::count)--`
• Keep the counts in a dedicated virtual base class.

The latter option (virtual base class) is probably the nicest; the compiler will enforce it's initialized once and only once, from the final ctor.

class counter {
    template struct perTypeCounter { static int count =  0; }
    int* count; // Points to perTypeCounter::count

    protected:
    template counter(MostDerived*) 
    {
        count = &perTypeCounter::count;
        ++*count;
    }
    ~counter() { --*count; }
};